Steerer tubes typically are arranged in combination with a head tube and bearing arrangement of a bicycle to allow the user to turn, or steer, the front wheel of the bicycle. In one arrangement, steerer tubes are configured as externally threaded steel tubes. Steering bearing adjustments are made using threaded locking systems, e.g., with adjusting nuts and lock nuts.
More recently, threaded steerer tubes have been replaced with threadless steerer tubes. Steering bearing systems for threadless steerers use fewer parts, resulting in simpler, lighter and easier to adjust assemblies. Typically, threadless steerer tubes require radial clearance between the steering bearing components and the steerer tube to facilitate assembly, allowing the components to be slid onto the steerer tube and then be fixed in place. In such a configuration, however, it may be necessary to connect the bearing, for example a rotating race component thereof, to the steerer tube. As shown for example in U.S. Pat. No. 5,096,770 to Rader, one solution for closing the clearance gap and securely fixing the rotating bearing race to the steerer tube uses a steering bearing top cap in combination with a compression ring. Rader discloses using a single action to simultaneously accomplish both a steerer gripping action and a bearing adjustment, although these two processes may be carried out as separate actions.
Recent trends in the bicycle industry have placed greater demands on the steering bearing systems. The steering bearing system of Rader, with a simple compression ring to grip the steerer, is no longer sufficient to meet the equipment and riding styles of these new trends.
One trend is a move to materials less robust than steel for use as steerer tubes. Initially, steel steerer tubes were replaced by aluminum tubes, which are softer and more susceptible to wear and abuse (e.g., rubbing, scrapes, scratches) than steel tubes. More recently, carbon fiber composites are being used for steerer tubes. While carbon fiber composites may be made extremely light and stiff in bending, they also may be more delicate and susceptible to abuse (dinged, scratched, worn, abraded) because the resins that hold the carbon fibers are softer than steel or aluminum. Typically, carbon fiber structures are excellent in tension and poor in compression. Accordingly, components designed to grip carbon fiber steerer tubes (e.g., compression rings, and stem clamps) may be more likely to damage the carbon fibers at the connection points.
For example, excessive compression forces may collapse or pinch carbon fiber composite steerer tubes such that the steerer tubes are more susceptible to failure in use. Indeed, carbon fiber composite structures may fail during normal loading/use conditions from an accumulation of damage over time (e.g., progressive delamination) or from damage sustained during a past overloading event (e.g., crash). Some steerer tube failures may result from excessive compressive force being applied by stem clamps. These steerer tube failures may cause a loss of steering control as the separation of the steerer tube at the stem connection location causes the handlebars and stem to detach. This trend towards using carbon fiber for steerer tubes is firmly established, so new steering bearing systems are needed that are able to adequately grip the steerer tube without applying excessive compressive gripping forces.
Another trend seen in the bicycle industry is a movement to newer styles of riding and equipment that place greater loads on the steering bearing assemblies. For example, suspension forks have been developed for aggressive downhill riding. These forks have increased the loads on steering bearings by using longer fork legs and shorter spacing between steerer bearings (i.e., shorter head tube and steerer tube). Over time the suspension fork legs have gotten longer (longer travel for bigger bumps) which creates a longer lever arm for impacts and braking forces from the wheel that push against the steering assembly bearings. As fork legs have gotten longer, the handle bar height has been maintained by shortening the frame head tube. The shorter head tube locates the steering bearings closer together. The closer spacing of the steering assembly bearings yields a shorter lever arm for the steerer bearings to resist forces from the fork legs. The two effects have combined (longer lever being resisted by a shorter lever) to increase the forces on the steering bearings by 2 or 3 times.
One of the deficiencies of Rader is that the compression ring of the headset is pushed radially into the steerer tube by axial compressive force applied to the steering bearing assembly. The common geometry of bearings and connections between steering bearing elements (tapered surfaces) may generate thousands of pounds of axial force in response to aggressive riding on modern bicycles. These large axial forces are carried through the compression ring, which may lead to damaging levels of compression on delicate materials like carbon fiber composites.
As shown in one prior art assembly (FIG. 3), a steerer tube 11 passes through head tube 12, cup 9, and bearing element 10. A compression ring 8, top cap 6, stem 4 and spacer(s) 5 slide onto steerer tube 11, with the stem 4 and spacer(s) 5 resting on top cap 6. A nut 3 is fixed in the steerer tube 11, with a stem cap 2 being placed on top of stem 4 and stem cap bolt 1 passing therethrough and threadably engaging the nut 3 so as to draw all of the aforementioned components together. As the stack of components is drawn together by stem cap bolt 1 and stem cap 2, the top cap 6 contacts compression ring 8. Compression ring 8 works in conjunction with the tapered surface of bearing element 10 to apply a radial compressive force 14 against steerer tube 11, rotationally connecting bearing element 10 to steerer tube 11 and radially fixing the steerer tube 11 within bearing element 10.
Once proper adjustment has been achieved, the clamping portion of stem 4 fixes the entire stack of components in place. At the same time, all components are fixed axially. At completion of installation, an axial force transmission path 13 is connected through the stacked components, including compression ring 8. Under the influence of an axial displacement for assembly, compression ring 8 will move such that its tapered surface slides on the corresponding tapered surface of bearing element 10, eventually engaging the steerer tube 11 so that further movement is constrained. In this manner, the axial forces applied to the compression ring results in a radial compressive force being applied to the steerer tube 11. The magnitude of the radial force developed is proportional to the applied axial force, and is dependent on the specific geometry of the mating parts, e.g., the angle of taper of compression ring 8 and bearing element 10. Moreover, changes in the magnitude of the axial compression force due to riding conditions (e.g., steering, jumping, hitting bumps, and stopping) correspondingly change the radial compressive force 14 applied to steerer tube 11. As riding a bicycle may produce repeated and large magnitude axial compressive forces, steerer tube 11 may be subjected to corresponding repeated and large magnitude radial compressive forces.
As such, a need remains for a steering bearing design that is capable of both securely gripping the steerer tube against high loads without applying excessive and possibly damaging compression forces to the steerer tube.